A polarizing plate having light transmission/shield function and a liquid crystal having a light switching function are both fundamental components of display devices such as liquid crystal displays (LCD). Application fields of the LCDs have been broadened from the earliest small machineries such as electronic calculators and clocks to note personal computers, word processors, liquid crystal projectors, liquid crystal televisions, car navigation system and interior or exterior-use measurement instrumentation, etc; and use conditions thereof have been widened from low temperature to high temperature, low humidity to high humidity, and low light amount to high light amount. In the circumstances, it has been desired to develop a polarizing plate having high polarization performance and excellent durability.
Currently, a polarizing film is manufactured by allowing iodine or a dichromatic dye to be contained in the polarizing film base such as a stretched/oriented film of a polyvinyl alcohol or a derivative thereof or a polyene-based film, which is formed by producing a polyene by removing hydrochloric acid from a polyvinyl chloride film or by removing water from a polyvinyl alcohol-based film, and orienting the polyene, with iodine or a dichromatic dye serving as a polarizing element. Of these, an iodine-based polarizing film using iodine as a polarizing element is excellent in polarization performance but weak against water and heat, and thus, has a problem in durability when it is used in high temperature/high humidity conditions for a long time. To improve the durability, treatment with formalin or an aqueous solution containing boric acid, and a method where a polymer film having a low water-vapor transmission rate is used as a protecting film have been considered; however, their effects are not always sufficient. On the other hand, a dye-based polarizing film using a dichromatic dye as a polarizing element is excellent in humidity resistance and heat resistance but generally insufficient in polarization performance, compared to the iodine-based polarizing film.
In a neutral color polarizing film formed by allowing a polymer film to adsorb several types of dichromatic dyes and orienting it, if light leakage (color leakage) occurs at a specific wavelength in a visible-light wavelength region in the state where two polarizing films are superimposed such that their orientation directions are crossed orthogonally (orthogonally crossed state), when the polarizing films are installed in a liquid crystal panel, the color tone of a liquid crystal display may sometimes change in a dark state. Then, when polarizing films are installed in a liquid crystal display device, in order to prevent discoloration of the liquid crystal display in a dark state caused by color leakage at a specific wavelength, transmissivity at the orthogonally crossed state (orthogonal-cross transmissivity) within the visible-light wavelength region must be uniformly reduced in a neutral color polarizing film formed by allowing a polymer film to absorb several types of dichromatic dyes and orienting it.
Furthermore, in the case of a color liquid-crystal projection type display, i.e., a color liquid-crystal projector, which uses a polarizing plate in a liquid crystal image forming section, an iodine-based polarizing plate having satisfactory polarization performance and displaying neutral gray has been used in the past. However, as described above, since the iodine-based polarizing plate uses iodine as a polarizing element, light resistance, heat resistance, and moist heat resistance are not sufficient. This is a problem. To overcome this problem, a neutral gray polarizing plate using a dye-based dichromatic pigment as a polarizer has come to be used. However, the neutral gray polarizing plate has the following problem. Since three primary color pigments are generally used in combination in order to uniformly improve the transmissivity and polarization performance within the entire visible-light wavelength region, the light transmissivity is low against the requirement for improving brightness as is the case of a color liquid crystal projector. To improve brightness, the intensity of a light source must be enhanced. To overcome this problem, three polarizing plates corresponding to three primary colors, in other words, for a blue channel, a green channel and a red channel, have come to be used.
However, brightness inevitably reduces for the reason that light is significantly absorbed by the polarizing plate and an image whose size is as small as 0.5 to 3 inches is enlarged to about several tens of inches to several hundreds of inches, and so forth. Therefore, a light source having a high brightness is used. In addition, a desire to further improve the brightness of a liquid crystal projector is still strong. As a result, needless to say, the intensity of a light source in use has more and more increased, and accordingly, light and heat applied to the polarizing plate have increased.
As the dye to be used in manufacturing the dye-based polarizing film as mentioned above, for example, water-soluble azo compounds described, for example, in PATENT DOCUMENT 1 to PATENT DOCUMENT 5, are mentioned.
However, conventional polarizing plates containing the water-soluble dyes as mentioned above have not yet satisfied needs in the market in view of polarization characteristics, absorption wavelength regions and color tone, etc. Furthermore, three polarizing plates corresponding to three primary colors of a color liquid crystal projector, more specifically, polarizing plates for a blue channel, a green channel and a red channel, having satisfactory brightness, polarization performance, durability in high temperature and high humid conditions, and further satisfactory light resistance to long-time exposure have not yet been obtained. Improvement thereof has been desired.